The ability of the oil industry to optimize the production of a reservoir relies on the possibility of evaluating the well effluent at regular intervals, in terms of quantity (flow rate) and of composition (the proportions of the various phases). This evaluation makes it possible to determine what corrective actions may need to be taken. Generally, measuring the flow rate of oil well effluents is a complex problem because the effluents are usually made up of three phases, and because of the changes in the flow conditions to which they are subject such as pressure, temperature, shape of pipes etc.
A traditional practice for measuring the flow rate of an oil well effluent is to separate the effluent into its component phases and to perform measurements on the phases separated in this way. This practice requires the installation of bulky separators on site. It also requires additional pipes to be put into place for connecting the separators. A preferred practice employs multiphase flow meters (MPFMs) that can measure the total flow rate and the individual flow rates or fractions of oil, gas and water of the effluent without prior separation. Numerous different types of MPFMs have been proposed. A description of such proposals can be found, for instance, in the publication SPE 28515 (SPE Annual Technical Conference, New Orleans, Sep. 25-28, 1994) by J. Williams entitled “Status of multiphase flow measurement research”. One type of well known MPFM is described in U.S. Pat. No. 6,405,604 issued to Berard et al. on Jun. 18, 2002. The MPFM described in the Berard patent combines Venturi and dual energy gamma fraction measurements. Variations of the above MPFM have been described in the following US patents: U.S. Pat. No. 6,265,713; U.S. Pat. No. 7,105,805; U.S. Pat. No. 7,240,568 all assigned to Schlumberger Technology Corporation.
In the oil industry, it is necessary to periodically monitor the flow rate of produced fluids obtained from each well of a group of wells located in the vicinity of each other. Such group of wells is commonly referred to as a “well pad”. In order to save on equipment, individual wells in a well pad are tested separately, one at a time, using one set of measuring devices. So for instance, it is preferred to use a single separator set up or a single multiphase flowmeter set up for servicing the wells in a well pad. To accomplish this, typically, a well switch directs the flow of the well that is selected to be tested to the separator or the flowmeter. The switch position is usually controlled by an automatic control system. However, because the flow rate and the fluid composition of the well effluents may differ significantly from different wells in a well pad it has been difficult to get accurate measurements, especially when using a single multiphase flowmeter. In fact, it is a rare situation when all wells from the well pad are of the same productivity such that the product fluids are all discharged at substantially the same pressure. Typically, there are wide disparities in well production and in the flow rates of the produced fluids from different wells in a well pad. For example, it is not unusual for the product flow rate of the lowest producing well to be about one-third or even less of the product flow rate of the highest producing well. Therefore, there exists a need to improve the accuracy of the MPFMs used to measure the flow rates of produced fluids from multiple wells in a well pad.